Photographic elements are coated on a wide variety of support materials, with the three major categories being plastics, paper and glass. Plastics are used as the support when a combination of transparency, strength, dimensional stability and light weight is needed. Useful plastics include cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate, polystyrene, polycarbonate and polyethylene terephthalate. Paper is the support of choice in situations where the physical property requirements are not too demanding, cost is a major factor or an opaque base is needed. Typically, photographic paper supports are coated on both sides with a thin layer of a polymeric resin such as polyethylene. Glass has the advantages of excellent dimensional stability and extreme flatness but is disadvantageous in that it is expensive, heavy and brittle. Photographic glass plates typically range in thickness from about 1 to about 10 millimeters and are graded by flatness. Those with the lowest degree of flatness are used in such applications as photomicrography and graphic arts, those with an intermediate degree of flatness in such applications as photofabrication, stereoplotters and aerial cameras, and those with the highest degree of flatness in such applications as high-precision stereoplotters, ballistic and aerotriangulation camera systems and special scientific investigations.
The coating of glass plates with photographic layers is a very demanding art since the layers must be extremely thin, highly uniform in thickness, and completely free from defects. One of the most effective techniques for accomplishing such coating is that described in U.S. Pat. No. 4,033,290, issued Jul. 5, 1977, in which a coating system based on wicking action is employed. In this system, an absorption wick having an arcuate surface is utilized in combination with a transfer wick that is in conforming contact with the arcuate surface. The difficulties involved in coating glass plates are described in this patent as follows:
"The coating of photographic glass plates is a very exacting coating operation in view of the stringent requirements that such plates must be capable of meeting. Thus, for example, to be used successfully for the coating of photographic glass plates the coating process must provide (1) complete coverage of the area which is to be coated, i.e., freedom from "skips" which would result in uncoated areas of even miniscule dimensions, (2) a coating layer which is extremely thin, (3) exactly uniform wet coverage, (4) freedom from coated material on the edges of the plate, (5) freedom from streaks or other coating defects, and (6) freedom from contamination with dust or other foreign materials. In addition the process must be capable of handling fragile sheets of glass without damage."
While the method and apparatus of the aforesaid U.S. Pat. No. 4,033,290 represent an important advance in the art of coating photographic glass plates and have proved highly successful, there are still serious concerns associated with their use. For example, there is a continuing need to still further reduce the number of streaks and other coating defects that occur. It would also be very desirable to be able to coat at much higher speeds than those that are attainable with a wick coater. Moreover, since photographic glass plates are needed in a very wide range of different sizes, there are inherent limitations in any coating operation that make it difficult to meet such needs. Thus, for example, a single coating line is not well suited to coating a very wide range of plates of differing widths, but the provision of multiple coating lines in order to have a separate line for each width desired is economically impractical. Yet another critical problem involved in the manufacture of photographic glass plates is the difficulty in providing edge-to-edge uniformity in the thickness of the silver halide emulsion layer.
As an alternative to coating the functional layers on the glass, photographic glass plates have been produced by adhesively securing a conventional radiation-sensitive photographic film to a sheet of glass by means of an adhesive layer interposed between the glass and the film support. The product resulting from such a process comprises a glass support having, in order on one surface thereof, the adhesive layer, the film support material--for example acetate film base or polyester film base--and the silver halide emulsion layer, as well as any other desired functional layers. This method of producing photographic glass plates avoids the problems involved in coating photographic coating compositions on glass. However, photographic film base is typically relatively thick, e.g., in the range of from about 0.1 to about 0.3 millimeters, and may exhibit many minor imperfections and, as a consequence, the optical characteristics of such photographic plates are seriously degraded.
A wide variety of laminate materials have been proposed for use in the photographic art and these laminates can be employed to protect the emulsion layer of photographic glass plates. Examples of such laminates include those disclosed in U.S. Pat. Nos. 4,077,830, 4,337,107, 4,378,392, 4,581,267 and 5,085,907. However, the photographic glass plates utilized in conjunction with such protective laminates have been made by conventional techniques as described hereinabove.
It is toward the objective of providing an improved way of manufacturing photographic glass plates, which provides much greater versatility in operation as well as enhanced product quality, that the present invention is directed.